Vertical-to-surface transmission electro-photonic device with ion implanted current control regions

ABSTRACT

The invention provides a vertical-to-surface transmission electro-photonic semiconductor device with a mesa structure of light reflective multiple layers in which the device includes a high resistive region for a carrier confinement. The high resistive region is formed by an ion-implantation of proton in a downward oblique direction during a rotation of a semiconductor substrate with use of a photo-resist mask whose horizontal width is larger than that of the mesa structure. The high resistive region defines a light emitting area of an active layer, an inverse circular truncated cone like definition of a top cladding region and a circular truncated cone like definition of a bottom cladding region. The oblique angle ion-implantation permits the top cladding region to be free from any exposure of the ion-implantation thereby an electrical resistance of the device is reduced.

BACKGROUND OF THE INVENTION

The invention relates to a semiconductor electro-photonic device, andmore particularly to a vertical-to-surface transmission electro-photonicdevice.

The stage of revolutions in the semiconductor material technology isentering into the age of developments in electro-photonic semiconductordevices including electronic and photonic devices for optical datatransmission and information processing. The value and importance inrealizations of high density and high speed parallel opticaltransmission, processing and computing application will be on theincrease in the semiconductor industry. The optical transmission devicessuch as light emitting devices for such the optical transmission andprocessing may be divided into two types. One is an edge transmissiondevice and the other is the vertical to surface transmission device. Thevertical-to-surface transmission electro-photonic device will receive agreat deal of attention increasingly as being suitable for atwo-dimensional integration which is able to permit the high density andhigh speed parallel optical transmission, processing and computingapplication. Such the vertical-to-surface transmission electro-photonicdevice as integrated in two-dimensional arrays is required to possessexcellent properties such as a high efficiency in the electronic-opticalpower conversion and a low electrical resistance for the above highdensity and high speed parallel optical transmission, processing andcomputing application. For the properties of the vertical-to-surfacetransmission electro-photonic device, failure of either the highefficiency in the electronic-optical power conversion or the lowelectrical resistance makes it difficult to accomplish the high densityand high speed parallel optical transmission, processing and computingapplication. For that reason, it seems essential that thevertical-to-surface transmission electro-photonic device is required topossess the above both properties or the high efficiency in theelectronic-optical power conversion and the low electrical resistance.So far as the inventor's knowledge, any of the conventionalvertical-to-surface transmission electro-photonic devices have, however,possessed insufficient properties both in the electronic-optical powerconversion efficiency and in the device resistance.

A typical structure of the conventional vertical-to-surface transmissionelectro-photonic device with a vertical cavity includes top and bottomreflective mirrors which sandwich intermediate layers. The intermediatelayers constitutes a double hetero structure which comprises an activelayer with a smaller refractive index and cladding layers with a largerrefractive index sandwiching the active layer. Such the double heterostructure with a compositional variation provides a potential well toconfine injection carriers and a refractive index discontinuity. Thecarrier confinement greatly enhances the utilization of injectioncarriers. The optical confinement causes the stimulated emission. Thelight or laser emission requires an injection of carriers or electronsand holes into the active layer. The injection of the carriers into theactive layer is able to cause the population inversion of of electronsin the active layer. The population inversion of electrons causes therecombination of electrons and holes which causes the spontaneousemission of photons or light. The light caused by the spontaneousemission is confined in the active layer with the smaller refractiveindex sandwiched by the cladding layers with the larger reflactiveindex. The confinement of the light caused by the spontaneous emissioncauses the stimulated downward transition of electrons which is able toemits photons or light. In the surface emitting layer, the propagationof the light emitted by the stimulated emission appears in a verticaldirection to a surface of the active layer. As described the above,since the light emitted from the active layer in the vertical directionis generated in the active layer by the spontaneous emission and thestimulated emission, an amount of the power of the light emitted fromthe active layer is defined by not only a magnitude of the carrierinjection into the active layer but also a size of the active region inthe vertical direction, namely a thickness of the active layer. Needlessto say, a large thickness of the active layer is able to permit of ageneration of a large power light emission in the direction vertical tosurface of the active layer. Generally, however, the active layer isrequired to be extremely thin as comprising a potential well structuresuch as a single or multiple quantum well structure.

A further enhancement of the stimulated downward transition of electronsor the stimulated emission requires a further optical confinement formuch more enhancement of the utilization of the light emitted from theactive layer. The further optical confinement of the light emitted inthe direction vertical to the surface of the active layer is made in thevertical cavity. The vertical cavity comprises a pair of reflectivemirrors such as distributed Bragg reflective mirrors for reflecting theemitted light in the direction vertical to the surface of the activelayer. The reflective mirrors are provided to sandwich the intermediatelayers having the potential well structure formed by the active layersandwiched by the cladding layers. The reflective mirrors are requiredto have a large reflectivity. The large reflectivity is provided by alarge refractive index discontinuity due to the compositional variationwhich appears on an interface between a small refractive indexsemiconductor layer and a large refractive index semiconductor layer.For example, to obtain a large reflectivity, each of the reflectivemirrors may comprise alternate laminations of large refractive indexsemiconductor layers such as AlAs layers and small refractive indexsemiconductor layers such as GaAs layers.

The large discontinuity of the reflactive index due to the compositionalvariation simultaneously provides a large energy band gap discontinuity.Then, the interface between the small refractive index semiconductorlayer and the large refractive index semiconductor layer necessarily hasa large energy band gap discontinuity in the direction across theinterface. The large energy band gap discontinuity necessarily providesa large potential barrier to carriers or electrons and holes across theinterface between the small and large refractive index semiconductorlayers, for example, the interface between the GaAs/AlAs layers. In theconventional surface emitting layer device with the vertical cavity, thecarriers are generally injected through the reflective mirror and thecladding layer into the active layer. In the conventional surfaceemitting laser device, a current pass of the injection carrier existsacross the interface between the small and large refractive indexsemiconductor layers such as the GaAs/AlAs layers in the reflectivemirrors. Then, the current pass of the injection carriers exists acrossthe potential barriers which appear on the interface between the smalland large refractive index semiconductor layers.

From the above descriptions, the reflective mirror with a largereflectivity suitable for optical confinement has a large potentialbarrier due to the large energy band gap discontinuity. In such thereflective mirror, the injection carrier necessarily experiences a largepotential barrier. This provides the enhancement of an electricalresistance of the current pass in the laser device. The enhancement ofthe electrical resistance causes problems with a requirement of a largeelectrical power of the current injection for obtaining the necessarylight emission as well as a generation of a large heat due to thecurrent of the injection carrier through the potential barrier. Eitherthe requirement of the large electrical power of the injection currentor the generation of the large heat constitutes a bar to realize thehigh density integration in the two dimensional arrays and the highspeed performance of the laser device with a lower power consumption.

To settle the above problems, the vertical-to-surface transmission layerdevice is required to possess a high efficiency in theelectronic-optical power conversion and a low electrical resistance forthe above high density and high speed parallel optical transmission,processing and computing application. The realization of the highefficiency in the electronic-optical power conversion requires the highreflectivity of the reflective mirrors which forms the vertical cavityto confine the stimulated emission light for a further enhancement ofthe stimulated downward transition of electrons or the stimulatedemission. On the other hand, the realization of the low electricalresistance requires a bypass of the currents of the injection carriersto avoid the interfaces of the small and large refractive indexsemiconductor layers having a large difference in the energy band gap sothat the injection carriers are permitted to be free from any largepotential barrier. The majority carriers of the p-type semiconductor areholes whose effective mass is larger than the effective mass ofelectrons serving as the carriers in the n-type semiconductor.

Particularly, the potential barrier to holes having the large effectivemass rather than that of electrons is a serious problem as providing alarge enhancement of the electrical resistance to the injectioncarriers. Namely, the potential barrier in the reflective mirror made ofp-doped semiconductor layers provides such serious problem.

To settle the above issue, surface emitting laser devices with any mesastructure were proposed in which a majority of the carriers injectedfrom the p-electrode flows on a bypass avoiding the potential barrieragainst holes caused by the energy band gap discontinuity in thereflective mirror made of the p-doped semiconductors as illustrated inFIGS. 4A and 4B. Such the mesa structure surface emitting laser deviceswere reported by Kurihara et al. in 1993 Japan J. Applied Physics Vol.32. pp. 604-608 as well as in Extended Abstracts of the 1992International Conference on Solid State Device and Materials, pp.598-600.

That conventional device or a vertical-to-surface transmissionelectro-photonic device with a mesa structure will hereinafter bedescribed in detail with reference back to FIG. 1. A substrate 1 for thevertical-to-surface transmission electro-photonic device with a verticalcavity is made of n-GaAs semiconductor compound. A bottom distributedBragg reflector mirror 2 comprises n-GaAs layers and n-AlAs layers whichare alternately laminated in which the lamination comprises nine periodsof the n-GaAs layer and n-AlAs layers, each of which has a thicknesscorresponding to a quater of a medium wavelength. The bottom distributedBragg reflector mirror 2 is formed on a top surface of the n-GaAssubstrate 1, although only a top pair of the n-GaAs layer and the n-AlAslayer is partially formed except in an n-electrode area. The n-GaAslayers and n-AlAs layers have a relatively large difference in thoserefractive index to serve as the distributed Bragg reflective mirror.The n-GaAs layers and n-AlAs layers with the large difference in therefractive index have also a relatively large difference in the energyband gap. A bottom cladding layer 3 made of n-Al₀.3 Ga₀.7 As is formedon the bottom distributed Bragg reflector mirror 2. An active layer 4 isformed on a top surface of the bottom cladding layer 3 in which theactive layer comprises an i-In₀.2 Ga₀.8 As layer which forms a singlequantum well structure. A top cladding layer 5 made of p-Al₀.3 Ga₀.7 Asis formed on a top surface of the active layer 4. A top distributedBragg reflector mirror 6 comprises p-GaAs layers and p-AlAs layers whichare alternately laminated in which the lamination comprises elevenperiods of the p-GaAs layers and p-AlAs layers, each of which has thethickness corresponding to a quater of the medium wavelength. The topdistributed Bragg reflector mirror 6 is formed on a predetermined areain a top surface of the top cladding layer 5 in which the predeterminedarea corresponds to a light emitting area in the device, notwithstandingonly a bottom pair of the p-GaAs layer and the p-AlAs layer is formed onan entire top surface of the top cladding layer 5. This results in thatthe top distributed Bragg reflector mirror 6 has a mesa structure. Thep-GaAs layers and p-AlAs layers have a relatively large difference inthose refractive indexes to serve as the distributed Bragg reflectivemirror. The p-GaAs layers and p-AlAs layers with the large difference inthe refractive index have also a relatively large difference in theenergy band gap.

As described above, the conventional vertical-to-surface transmissionelectro-photonic device has a vertical cavity which comprises the topand bottom distributed Bragg reflector mirrors 6 and 2 in which thereflective mirrors 6 and 2 sandwich intermediate layers comprising theactive layer 4 and the top and bottom cladding layers 5 and 3. Moreover,the conventional vertical-to-surface transmission electro-photonicdevice has not only the above mesa structure of the top distributedBragg reflector mirror 6 but also high resistive regions 12 comprisingproton-implanted regions. As illustrated in FIG. 1, the high resistiveregions 12 are partially formed except in the light emitting area in theintermediate layers, namely the active layer 5 and the top and bottomcladding layers 5 and 3. It is desired that the high resistive regions12 are tapering off as illustrated in FIG. 1, although it is difficultto form so. A horizontal distance between the high resistive regions 12is equal to a length of the light emitting area under the mesa structureof the top distributed Bragg reflector 6.

A p-electrode 14 is formed to cover the mesa structure of the topdistributed Bragg reflector mirror 6 and its adjacent portions. Ann-electrode 13 is formed on the n-electrode region without theintermediate layers in which the bottom distributed Bragg reflectormirror 2 is exposed.

The description will subsequently be directed to fabrication processesfor the above conventional vertical-to-surface transmissionelectro-photonic device.

With reference to FIG. 2A, the n-GaAs substrate 1 is prepared and then-GaAs layers and the n-AlAs layers are epitaxially and alternatelygrown by molecular beam epitaxy on the top surface of the n-GaAssubstrate 1 until the nine periods of the alternations of the n-GaAslayers and the n-AlAs layers are formed to serve as the bottomdistributed Bragg reflector mirror 2. The n-Al₀.3 Ga₀.7 As epitaxiallayer serving as the cladding layer 3 is grown by molecular beam epitaxyon the top surface of the bottom distributed Bragg reflector mirror 2.The non-doped In₀.2 Ga₀.8 As epitaxial layer serving as the active layer4 is grown on the top surface of the bottom cladding layer 3 bymolecular beam epitaxy. The p-Al₀.3 Ga₀.7 As epitaxial layer serving asthe top cladding layer 5 is grown on the active layer 4 by molecularbeam epitaxy. The p-GaAs layers and the p-AlAs layers are epitaxiallyand alternately grown by molecular beam epitaxy on the top surface ofthe top p-doped cladding layer 5 until the eleven periods of thealternations of the p-GaAs layers and the p-AlAs layers are formed toserve as the p-doped top distributed Bragg reflector mirror 6 thereby avertical-to-surface emitting laser substrate 7 is completed.

With reference to FIG. 2B, a photo-resist film is formed on a topsurface of the p-doped top distributed Bragg reflector 6 in thevertical-to-surface emitting laser substrate 7. A patterning for thephoto-resist film is accomplished so that the photo-resist film ispartially removed and a photo-resist pattern remains only in the lightemitting area in which the mesa structure of the p-doped top distributedBragg reflector will be formed.

With reference to FIG. 2C, except for the bottom one or two periods ofthe p-GaAs layer and the p-AlAs layer, the p-doped top distributed Braggreflector mirror 6 is selectively removed by a reactive ion-etching 9using a chlorine gas and the photo-resist pattern 8 so that the mesastructure 10 of the p-doped top distributed Bragg reflector 6 is definedin the light emitting area covered by the photo-resist mask 8.

With reference to FIG. 2D, an ion-implantation of proton in a verticaldirection is accomplished by use of the photo-resist mask 8 so thatproton is implanted into the epitaxial intermediate layers except in thelight emitting area covered by the photo-resist mask 8. This results inthat the epitaxial intermediate layers except in the light emitting areacovered by the photo-resist mask 8 become high resistive regions 12 ofthe proton implanted regions. So far as the proton or other impurityimplantations in the vertical direction are concerned, the lightemitting area is defined by the photo-resist mask 8 used for the protonimplantation. In the above processes, the photo-resist mask 8 was usednot only in the dry etching process to form the mesa structure 10 of thep-doped top distributed Bragg reflector mirror 6 but also in the protonimplantation process to form the high resistive regions 12. For thosereasons, the size of the photo-resist mask 8 is able to define not onlya horizontal size of the mesa structure 10 of the p-doped topdistributed Bragg reflector mirror 6 but also a size of the lightemitting area surround by the proton implanted regions as the highresistive regions 12.

With reference to FIGS. 2E and 2F, the photo-resist mask 8 is removed,after that a photo-resist film is formed on an entire surface of thedevice to cover the mesa structure 10 of the p-doped top distributedBragg reflector mirror 6 and peripheral flat surface of the device. Thephoto-resist film is patterned so that part of the photo-resist filmonly in the n-electrode region is removed to serve as a photo-resistmask. A selective wet etching is accomplished by use of the photo-resistmask so that the bottom pair of the p-GaAs/AlAs layers involved in thep-doped top distributed Bragg reflector mirror 6 is partially etched butonly in the n-electrode region. Subsequently, the p-doped top claddinglayer 5, the active layer 4 and the bottom cladding layer 3 arepartially etched in turn in the n-electrode region. Further, a top pairof the n-GaAs/AlAs layers involved in the n-doped bottom distributedBragg reflector 2 is partially etched in the n-electrode region. Theabove wet etching is accomplished until at least the n-doped bottomdistributed Bragg reflector mirror 2 is exposed.

With reference to FIG. 2G, an n-electrode is formed on the exposedsurface in the n-electrode region of the n-doped bottom distributedBragg reflector mirror 2. A p-electrode is formed to cover both the mesastructure 10 of the p-doped top distributed Bragg reflector mirror aswell as flat portions adjacent to the mesa structure 10 of thep-GaAl/AlAs layers involved in the p-doped top distributed Braggreflector mirror 6 thereby the fabrication processes for theconventional vertical-to-surface transmission electro-photonic device iscompleted.

As to the ion-implantation of proton in the vertical direction to formthe proton implanted regions as the high resistive region, ideally, theproton is implanted at a predetermined energy such that a protonconcentration profile in the vertical direction has a peak value in theactive layer 4 thereby a cross sectional definition of the protonimplanted region is tapering off toward the light emitting area. So faras the proton implantation or other impurity implantation in thevertical direction is concerned, it is, however, difficult to obtain theabove tapering cross sectional definition of the proton implantedregion. Actually, somewhat of the implanted protons is subject toremaining at portions adjacent to the surface exposed to the protonimplantation around the mesa structure 10 of the p-doped top distributedBragg reflector mirror 6. For that reason, in the area adjacent to themesa structure 10, the bottom pair in the p-doped top distributed Braggreflector mirror 6 and the p-doped top cladding layer 5 become resistiveregions.

In the p-side of the device, it is important to suppress the carrier toshow a downward current through the mesa structure 10 including thealternating laminations of the GaAs/AlAs layers which forms relativelylarge potential barriers due to relatively large energy band gapdiscontinuities. The large energy band gap discontinuity is caused bythe compositional discontinuity of the GaA/AlAs layers. Thecompositional discontinuity is necessary to permit the largereflectivity possessed by the p-type top distributed Bragg reflector ofthe mesa structure 10.

As described above, it could be understood that the mesa structure 10 ofthe p-type top distributed Bragg reflector has a large electricalresistance to the majority carriers or holes. For that reason, a largepart of the carriers or holes is supplied from a peripheral portion ofthe p-electrode 14 around the mesa structure 10 and subsequently flowthrough the top pair of the p-GaAs/AlAs semiconductor layers and thep-type top cladding layer 5 and then injected into the active layer 4.At this time, the transmitted carriers are subjected to the horizontalcarrier confinement into the light emitting area by the high resistiveregions 12 of the proton implanted regions with the tapering structure.

The conventional device is, however, engaged with the followingproblems. As described above, a large part of the injection carriersflows from the peripheral portion of the mesa structure 10 toward thelight emitting area in the active layer 4 and receives the carrierconfinement by the high resistive regions 12 of the proton implantedregions. Namely, a large part of the injection carriers flows part ofthe bottom pair of the p-GaAs/AlAs and the p-cladding layer 5 over thetapering portions of the high resistive regions 12 around the mesastructure 10. However, as described above, the actual protonimplantation in the vertical direction necessarily permits somewhat ofthe protons not to reach the active layer 4 and thus to remain in thepart of the p-GaAs/AlAs layers and the p-cladding layer 5 over thetapering portions of the high resistive regions 12 around the mesastructure 10.

From the above, it could readily be appreciated that the part of thep-GaAs/AlAs layers and the p-cladding layer 5 over the tapering portionsof the high resistive regions 12 is a somewhat high resistive region notso much as the high resistive regions 12. The majority of the carriersor holes flows through the somewhat resistive part the p-GaAs/AlAslayers and the p-cladding layer 5 over the tapering portions of the highresistive regions 12. This provides the effective and actual increase ofthe electrical resistance of the device. This makes it impossible toprovide a low electrical resistance which is one of the most importantfactors for the vertical-to-surface light emitting device.

To settle the above serious problem, an alternative proposal for thevertical-to-surface light emitting device have been known in the art, towhich the present invention pertains. FIG. 3 illustrates a crosssectional structure of the another conventional vertical-to-surfacelight emitting device with the vertical cavity. It could readily beappreciated that the another conventional vertical-to-surface lightemitting device as illustrated in FIG. 3 has a remarkable difference inthe structure from the conventional vertical-to-surface light emittingdevice as illustrated in FIG. 1. The remarkable difference appears in ahorizontal size of the active layer 4 defined by the tapering portionsof the high resistive regions 12 of the proton implanted regions or in ahorizontal distance between the high resistive regions 12 of the protonimplanted regions. While the conventional vertical-to-surface lightemitting laser as illustrated in FIG. 1 has the same horizontal distancebetween the high resistive regions 12 as the width of the mesastructure, the another conventional vertical-to-surface light emittinglaser as illustrated in FIG. 3 has a larger horizontal distance D2between the high resistive regions 12 than the width D1 of the mesastructure 10. Such a larger horizontal distance D2 between the highresistive regions 12 than the width D1 of the mesa structure 10 is ableto permit the device to be free from the above problem as to the highelectrical resistance. In the alternative conventionalvertical-to-surface light emitting device, the majority of the carriersor holes supplied from the flat and peripheral part of the p-electrode14 around the mesa structure 10 is able to flow through proton freep-type epitaxial layers into the active layer. Almost no carriers orholes flows through the somewhat resistive part of the p-type epitaxiallayers over the tapering portions of the proton implanted high resistiveregions 12 into the active layer 4.

Such the vertical-to-surface light emitting device with the largercarrier injection area of the active layer 4 illustrated in FIG. 3 is,however, engaged with the following disadvantage. The injection carriersreceive almost no carrier lateral confinement by the proton implantedhigh resistive regions 12. This results in a low current density of theinjection carriers or holes into the active layer 4 in the enlargedcarrier injection area with the wide width of D2. The low currentdensity of the injection carriers into the enlarged carrier injectionarea of the active layer 4 results in a reduction of the power of thelight or laser emitted from the active layer. A strong light or laseremission requires a further large injection carrier which leads to areduction in the electric-optical conversion efficiency. Suchvertical-to-surface light emitting device including the enlarged carrierinjection area of the active layer is unavoidably engaged with thedisadvantage in the low efficiency in the electronic-optical conversion.

It has therefore been required to develop a novel vertical-to-surfacetransmission electro-photonic device which possesses not only a highefficiency in the electronic-optical conversion but also an extremelylow electrical resistance.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea novel vertical-to-surface transmission electro-photonic device freefrom any problems as mentioned in the above.

It is an object of the present invention to provide a novelvertical-to-surface transmission electro-photonic device which possessesa reduced electrical resistance.

It is a further object of the present invention to provide a novelvertical-to-surface transmission electro-photonic device which possessesa property of a high electronic-optical conversion efficiency.

It is a further more object of the present invention to provide a novelvertical-to-surface transmission electro-photonic device which possessesa property of a low power consumption.

It is a still further object of the present invention to provide a novelvertical-to-surface transmission electro-photonic device which issuitable for high density two-dimensional arrays for parallel opticalprocessing and transmissions.

It is yet a further more object of the present invention to provide anovel vertical-to-surface transmission electro-photonic device whichshows high speed optical processing and transmissions.

It is an additional object of the present invention to provide a novelmethod for fabricating a vertical-to-surface transmissionelectro-photonic device free from any problems as mentioned in theabove.

It is another object of the present invention to provide a novel methodfor fabricating a vertical-to-surface transmission electro-photonicdevice which possesses a reduced electrical resistance.

It is still another object of the present invention to provide a novelmethod for fabricating a vertical-to-surface transmissionelectro-photonic device which possesses a property of a highelectronic-optical conversion efficiency.

It is yet another object of the present invention to provide a novelmethod for fabricating a vertical-to-surface transmissionelectro-photonic device which possesses a property of a low powerconsumption.

It is a still additional further object of the present invention toprovide a novel method for fabricating a vertical-to-surfacetransmission electro-photonic device which is suitable for high densitytwo-dimensional arrays for parallel optical processing andtransmissions.

It is yet a further more object of the present invention to provide anovel method for fabricating a vertical-to-surface transmissionelectro-photonic device which shows high speed optical processing andtransmissions.

The above and objects, features and advantages of the present inventionwill be apparent from the following descriptions.

The present invention provides a vertical-to-surface transmissionelectro-photonic semiconductor device. The device includes asemiconductor substrate, a first light reflective means formed on thesemiconductor substrate for reflecting a light propagated in a verticaldirection, a first cladding region of a first conductivity type beingselectively formed on a part of the first reflective means in which thefirst cladding region has a definition of a circular truncated cone, anactive layer defined within a light emitting area and being formed on atop surface of the first cladding layer, a second cladding layer of asecond conductivity type formed on the active layer in the lightemitting area in which the second cladding region has a definition of aninverse circular truncated cone, a carrier confinement means comprisinga high resistive region formed to define the light emitting area of theactive layer, the circular truncated cone definition of the firstcladding region and the inverse circular truncated cone definition ofsecond cladding region, a second light reflective means having a mesastructure selectively formed on a part of a top surface of the secondcladding region for reflecting a light propagated in a verticaldirection and the predetermined area approximately corresponding to thelight emitting area in a plane view, a first electrode provided on apart of the device under the active layer except in the high resistiveregion, and a second electrode formed to cover at least a peripheral ofthe mesa structure.

The inverse circular truncated cone definition of the second claddingregion has a side surface with an oblique angle of approximately 14° atan included angle to a surface of the active layer. The high resistiveregion of the carrier confinement means is formed by a selectiveion-implantation with use of a mask having a larger horizontal size thana horizontal size of the mesa structure at an oblique anglecorresponding to an oblique angle of a side surface of the inversecircular truncated cone definition of the second cladding layer so thatthe entire of the second cladding region having the inverse circuittruncated cone definition is completely free from the ion-implantation.

The oblique angle and the horizontal size of the mask for theion-implantation are respectively defined to comply with the followingconditions:

    B=(2D(W.sub.1 +W.sub.2)/W.sub.2)+A; and

    β=tan.sup.-1 (W.sub.2 +D)

where B is the horizontal size of the mask, β is the oblique angle beingan included angle to the horizontal line, W₁ is a vertical distancebetween a bottom surface of the mask and the top surface of the secondcladding region, W₂ is a depth of the active layer from the top surfaceof the second cladding region, A is a horizontal size of the activelayer formed within the light emitting area defined by the highresistive region and D is a horizontal distance between an edge of thetop surface of the second cladding region and an edge of the lightemitting area of the active layer.

The ion-implantation is accomplished during a rotation of thesemiconductor substrate around a vertical center axis of the device.

The mesa structure of the second light-reflective means either comprisesa rectangular-shaped section or comprises a bottom trapezoid-shapedsection and a top rectangular-shaped section.

Alternatively, the first cladding region may have a definition of atruncated pyramid and the second cladding region may have a definitionof an inverse truncated pyramid. In this case, the high resistive regionis formed to define the light emitting area of the active layer, thetruncated pyramid definition of the first cladding region and theinverse truncated pyramid definition of second cladding region.

The present invention also provide a novel method of fabricating avertical-to-surface transmission electro-photonic semiconductor device.The novel fabrication method comprises the steps of growing first lightreflective multiple layers on a top surface of a semiconductorsubstrate, growing a first cladding layer of a first conductivity typeon a top surface of the first light reflective multiple layers, growingan active layer on a top surface of the first cladding layer, growing asecond cladding layer of a second conductivity type on a top surface ofthe active layer, selectively growing second light reflective multiplelayers having a mesa structure in a predetermined area on a top surfaceof the second cladding layer, forming a photo-resist mask on a top ofthe mesa structure of the second light reflective multiple layers inwhich the photo-resist mask has a larger horizontal size than ahorizontal size of the mesa structure, selectively subjecting a part ofthe device to an ion-implantation in a predetermined downward obliquedirection with use of the photo-resist mask to form a high resistiveregion which defines an ion-free region having an inverse circulartruncated cone definition of the second cladding layer, an ion-freelight emitting area of the active layer and an ion-free region having acircular truncated cone definition of the first cladding layer in whichthe predetermined downward oblique direction for the ion-implantationdefines a slop of a side surface of the ion-free inverse circulartruncated cone definition region of the second cladding layer, removingthe photo-resist mask, and providing a first electrode on a part of thesemiconductor substrate and a second electrode on at least a top surfaceof the ion-free inverse circular truncated cone region of the secondcladding layer.

The predetermined downward oblique direction for the ion-implantation isdefined to have an included angle of approximately 14° to a surface ofthe active layer. The downward oblique direction and the horizontal sizeof the mask for the ion-implantation process are respectively defined tocomply with the following conditions:

    B=(2D(W.sub.1 +W.sub.2)/W.sub.2)+A; and

    β=tan.sup.-1 (W.sub.2 +D)

where B is the horizontal size of the mask, β is the oblique angle beingan included angle to the horizontal line, W₁ is a vertical distancebetween a bottom surface of the mask and the top surface of the secondcladding region, W₂ is a depth of the active layer from the top surfaceof the second cladding region, A is a horizontal size of the activelayer formed within the light emitting area defined by the highresistive region and D is a horizontal distance between an edge of thetop surface of the second cladding region and an edge of the lightemitting area of the active layer. The ion-implantation is accomplishedduring a rotation of the semiconductor substrate around a verticalcenter axis of the device.

The photo-resist mask for the ion-implantation is formed by thefollowing method comprising the steps of applying a photo-resistmaterial on an entire surface of the device to cover the mesa structureof the second light reflective multiple layers, selectively removing thephoto-resist material by etching to expose a top surface of the mesastructure of the second light reflective multiple layers, formingnegative photo-resist film on the exposed top surface of the mesastructure and on an exposed surface of the remaining photo-resistmaterial, and subjected the negative photo-resist film to patterning toform the photo-resist mask.

The mesa structure of the second light reflective multiple layerscomprises a rectangular-shaped section being formed by the followingmethod comprising the steps of selectively forming a mask pattern in apredetermined area on a top surface of the second light reflectivemultiple layers, selectively removing a part of the second lightreflective multiple layers by an etching with use of the mask pattern,and removing the mask pattern.

Alternatively, the mesa structure of the second light reflectivemultiple layers comprises a bottom trapezoid-shaped section and a toprectangular-shaped section being formed by the following methodcomprising the steps of selectively forming mask pattern in apredetermined area on a top surface of the second light reflectivemultiple layers, selectively removing apart of the second lightreflective multiple layers by an etching with use of the mask pattern,and subjecting the remaining part of the second reflective multiplelayers to a wet etching. The photo-resist mask for the ion-implantationis carried out with use of the mask pattern used in the mesa structureformation processes.

Alternatively, a part of the device may be subjected to anion-implantation in a predetermined downward oblique direction with useof the photo-resist mask to form a high resistive region which definesan ion-free region having an inverse truncated pyramid definition of thesecond cladding layer, an ion-free light emitting area of the activelayer and an ion-free region having a truncated pyramid definition ofthe first cladding layer in which the predetermined downward obliquedirection for the ion-implantation defines a slop of a side surface ofthe ion-free inverse truncated pyramid definition region of the secondcladding layer. In this case, the ion-implantation is accomplishedduring a stationary state of the semiconductor substrate and isdiscontinued during a rotation of the semiconductor substrate around avertical center axis of the device by a predetermined angle.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will hereinafter fully bedescribed in detail with reference to the accompanying drawings.

FIG. 1 is a fragmentary cross sectional elevation view illustrative ofthe structure of the conventional vertical-to-surface transmissionelectro-photonic device.

FIGS. 2A to 2G are fragmentary cross sectional elevation viewsillustrative of sequential steps involved in the conventional method forfabricating the conventional vertical-to-surface transmissionelectro-photonic device.

FIG. 3 is a fragmentary cross sectional elevation view illustrative ofthe structure of the other conventional vertical-to-surface transmissionelectro-photonic device having the enlarged carrier injection area ofthe active layer.

FIG. 4A is a cross sectional elevation view illustrative of thestructure of the other conventional vertical-to-surface transmissionelectro-photonic device having the double mesa structure.

FIG. 4B is a modeled perspective view illustrative of the structure ofthe conventional vertical-to-surface transmission electro-photonicdevice illustrated in FIG. 4A.

FIG. 5 is a fragmentary cross sectional elevation view illustrative of astructure of a novel vertical-to-surface transmission electro-photonicdevice of a first embodiment according to the present invention.

FIGS. 6A to 6J are fragmentary cross sectional elevation viewsillustrative of sequential steps involved in a novel method forfabricating a novel vertical-to-surface transmission electro-photonicdevice of a first embodiment according to the present invention.

FIG. 7 is a fragmentary cross sectional elevation view illustrative of arelationship between a horizontal mask size and an oblique angle for anion-implantation in a fabrication process for a high resistive regioninvolved in a novel vertical-to-surface transmission electro-photonicdevice according to the present invention.

FIG. 8 is a fragmentary cross sectional elevation view illustrative of astep involved in a novel method for fabricating a novelvertical-to-surface transmission electro-photonic device of a secondembodiment according to the present invention.

FIG. 9 is a fragmentary cross sectional elevation view illustrative of astructure of a novel vertical-to-surface transmission electro-photonicdevice of a second embodiment according to the present invention.

FIGS. 10A to 10 H are fragmentary cross sectional elevation viewsillustrative of sequential steps involved in a novel method forfabricating a novel vertical-to-surface transmission electro-photonicdevice of a second embodiment according to the present invention.

FIG. 11 is another fragmentary cross sectional elevation view of astructure of a novel vertical-to-surface transmission electro-photonicdevice showing the highly sensitive region 12 and the region 15 in whichthe implanted impurities came to rest.

PREFERRED EMBODIMENTS OF THE INVENTION

A first embodiment according to the present invention provides a novelvertical-to-surface transmission electro-photonic device with a mesastructure, a novel structure of which will hereinafter be described indetail with reference to FIG. 5.

A novel vertical-to-surface transmission electro-photonic device has asemiconductor substrate which is made of an n-GaAs semiconductorcompound. A bottom distributed Bragg reflector mirror 2 is provided on atop surface of said n-GaAs semiconductor substrate 1. The bottomdistributed Bragg reflector mirror 2 comprises n-GaAs layers and n-AlAslayers which are alternately laminated. The alternating lamination may,for example, comprise five periods of the n-GaAs layers and n-AlAslayers, each of which has a thickness corresponding to a quater of amedium wavelength. The n-GaAs layers and n-AlAs layers have a relativelylarge difference in those refractive indexes due to those compositionaldiscontinuity to serve as the distributed Bragg reflective mirror forreflecting a light or laser propagated in a vertical direction. Then-GaAs layers and n-AlAs layers with the large difference in therefractive index have also a relatively large discontinuity in theenergy band gap due to the compositional discontinuity.

A bottom cladding region 3 made of n-Al₀.3 Ga₀.7 As is provided on a topsurface of the bottom distributed Bragg reflector mirror 2. Asillustrated in FIG. 5, the bottom n-Al₀.3 Ga₀.7 As cladding region has adefinition of a circular truncated cone whose horizontal size ordiameter is gradually and proportionally minimized toward an upperdirection. The circular truncated cone definition of the bottom claddingregion is completely free from any exposure of an ion-implantation for aformation of a high resistive region for a lateral carrier confinement.Thus, the bottom cladding region having the circular truncated conedefinition has an extremely low electrical resistance through which anycarrier or electrons are able to flow without any experience of apotential barrier.

An active layer 4 is formed on a top surface of the bottom claddingregion 3. The active layer 4 comprises a i-In₀.2 Ga₀.8 As layer whichforms a quantum well structure. The active layer 4 is defined in ahorizontal carrier injection area which corresponds to the top surfacearea of the bottom cladding region 3.

A top cladding region 5 made of p-Al₀.3 Ga₀.7 As is provided on a topsurface of the active layer defined in the carrier injection area. Asillustrated in FIG. 5, the top p-Al₀.3 Ga₀.7 As cladding region 5 has adefinition of an inverse circular truncated cone whose horizontal sizeor diameter is gradually and proportionally enlarged toward the upperdirection. The inverse circular truncated cone definition of the topcladding region 5 is completely free from any exposure of anion-implantation for a formation of a high resistive region for alateral carrier confinement. Notwithstanding in the prior art the topcladding layer around the mesa structure is unavoidably exposed to theion-implantation thereby the top cladding layer over the taperingportion of the high resistive region unavoidably has a somewhat highelectrical resistivity. In contrast, in the first embodiment accordingto the present invention, the ion-free top cladding region having theinverse truncated cone definition is able to have an extremely lowelectrical resistivity. Since the bottom cladding region having thecircular truncated cone definition has an extremely low electricalresistance, any carrier or holes are able to flow without any experienceof a potential barrier. Since holes as the carrier in the p-type topcladding region 5 have a much larger effective mass than an effectivemass of electrons as carriers in the n-type bottom cladding region 3,the extremely low resistivity of the top cladding region having theinverse circular truncated cone definition is much more important forsecuring a desired low electrical resistance of the device. This is veryimportant to simultaneously secure both a high efficiency in theelectronic-optical conversion and a very low device resistance.

An oblique angle of a side surface of the inverse circular truncatedcone definition may be, for example, approximately 14° at the includedangle to the level surface or the surface of the active layer 4. Theoblique angle of the side surface in the inverse circular truncated conedefinition of the top cladding region is defined by an oblique angle ofan oblique direction ion-implantation for a formation of a highresistive region 12 for a lateral carrier confinement. The highresistive region may comprises a proton implanted region. The highresistive region 12 acting for the lateral carrier confinement isprovided to define the inverse circular truncated cone definition of thep-type top cladding region 5, the carrier injection area of the activelayer 4 and the circular truncated cone definition of the n-type bottomcladding region 3.

Two pairs of p-GaAs/AlAs layers may be provided on the top surface ofthe p-type top cladding region 5 having the inverse circular truncatedcone definition. The two pairs of the p-GaAs/AlAs layers have anexternal definition like a flat inverse circular truncated cone smoothlyunited with the inverse circular truncated cone definition of the p-typetop cladding region 5. The p-type top cladding region 5 and the twopairs of the p-GaAs/AlAs layers constitute a united inverse circulartruncated cone as illustrated in FIG. 5. Then, a side surface of theflat inverse circular truncated cone definition is defined by the highresistive region 12.

Alternatively, a single pair of p-GaAs/AlAs layers may be provided onthe top surface of the p-type top cladding region 5 having the inversecircular truncated cone definition whereas an illustration is omitted.The single pair of the p-GaAs/AlAs layers has an external definitionlike the inverse circular truncated cone smoothly united with theinverse circular truncated cone definition of the p-type top claddingregion 5. The p-type top cladding region 5 and the single pair of thep-GaAs/AlAs layers constitute a united inverse circular truncated coneas illustrated in FIG. 5.

Alternatively, no pair p-GaAs/AlAs layers may be provided on the topsurface of the p-type top cladding region 5 having the inverse circulartruncated cone definition whereas an illustration is omitted.

Whereas in the above descriptions the above n-type bottom and p-type topcladding regions 3 and 5 have the circular truncated cone and theinverse circular truncated cone definitions, modifications of thedefinitions thereof are acceptable. For example, the externaldefinitions of the n-type bottom and the p-type top cladding regions maybe a truncated pyramid and an inverse truncated pyramid respectively,for example, a quadrilateral truncated pyramid and an inversequadrilateral truncated pyramid.

A horizontal size or diameter of the inverse truncated pyramid of thep-type top cladding region 5 is gradually and proportionally enlargedtoward the upper direction. The inverse truncated pyramid definition ofthe top cladding region 5 is completely free from any exposure of anion-implantation for a formation of a high resistive region for alateral carrier confinement. Notwithstanding in the prior art the topcladding layer around the mesa structure is unavoidably exposed to theion-implantation thereby the top cladding layer over the taperingportion of the high resistive region unavoidably has a somewhat highelectrical resistivity. In contrast, the modification of the firstembodiment according to the present invention, the ion-free top claddingregion having the inverse truncated cone definition is able to have anextremely low electrical resistivity. Since the bottom cladding regionhaving the truncated pyramid definition has an extremely low electricalresistance, any carrier or holes are able to flow without any experienceof a potential barrier. Since holes as the carrier in the p-type topcladding region 5 have a much larger effective mass than an effectivemass of electrons as carriers in the n-type bottom cladding region 3,the extremely low resistivity of the top cladding region having theinverse truncated pyramid definition is much more important for securinga desired low electrical resistance of the device. This is veryimportant to simultaneously secure both a high efficiency in theelectronic-optical conversion and a very low device resistance.

An oblique angle of a side surface of the inverse truncated pyramiddefinition may be, for example, approximately 14° at the included angleto the level surface or the surface of the active layer 4. The obliqueangle of the side surface of the inverse truncated pyramid definition ofthe top cladding region is defined by an oblique angle of an obliquedirection ion-implantation for a formation of a high resistive region 12for the lateral carrier confinement. The high resistive region maycomprises a proton implanted region. The high resistive region 12 actingfor the lateral carrier confinement is provided to define the inversetruncated pyramid definition of the p-type top cladding region 5, thecarrier injection area of the active layer 4 and the truncated pyramiddefinition of the n-type bottom cladding region 3.

Two pairs of p-GaAs/AlAs layers may be provided on the top surface ofthe p-type top cladding region 5 having the inverse truncated pyramiddefinition. The two pairs of the p-GaAs/AlAs layers have an externaldefinition like a flat inverse truncated pyramid smoothly united withthe inverse truncated pyramid definition of the p-type top claddingregion 5. The p-type top cladding region 5 and the two pairs of thep-GaAs/AlAs layers constitute a united inverse truncated pyramid asillustrated in FIG. 5. Then, a side surface of the flat inversetruncated pyramid definition is defined by the high resistive region 12.

The description will hereinafter be back to the first embodiment fromthe modifications thereof.

A top distributed Bragg reflector mirror 6 comprises p-GaAs layers andp-AlAs layers which are alternately laminated. The alternatinglamination may, for example, comprise ten periods of the p-GaAs layersand p-AlAs layers, each of which has the thickness corresponding to aquater of the medium wavelength. The top distributed Bragg reflectormirror 6 is formed on a predetermined area in a top surface of the twopairs p-GaAs/AlAs layers having the inverse circular truncated conedefinition in which the predetermined area corresponds to the lightemitting area or the carrier injection area of the active layer 4 thetop cladding layer 5 in a plane view. This results in that the topdistributed Bragg reflector mirror 6 has a mesa structure. The p-GaAslayers and p-AlAs layers have a relatively large difference in thoserefractive indexes due to a large compositional discontinuity to serveas the distributed Bragg reflective mirror. The p-GaAs layers and p-AlAslayers with the large difference in the refractive index have also arelatively large energy band gap discontinuity due to the largecompositional discontinuity.

As modifications of the first embodiment, the above number of theperiods for the p-type bottom and the p-type top distributed Braggreflector mirrors may be changeable to much various conditions.

A p-electrode 14 is formed to cover not only the mesa structure 10 ofthe top distributed Bragg reflector mirror 6 and its adjacent portions.The p-electrode 14 is provided to cover at least the exposed top surfaceof the two pairs of the p-GaAs/AlAs layers 6 in which the exposedsurface is not covered by the mesa structure of the top distributedBragg reflector mirror 6. The p-electrode 14 is provided further tocover an edge portion of the top surface of the high resistive region12. An n-electrode 13 is selectively provided at a peripheral positionon a bottom surface of the n-GaAs substrate 1 so that the n-electrode 13does not cover any light or laser beam emitting area. Alternatively,n-electrode 13 may be designed to have a window through which the lightor laser beam is able to be transmitted.

The description of the first embodiment and some modifications thereofwill subsequently be directed to fabrication processes for the abovenovel vertical-to-surface transmission electro-photonic device.

With reference to FIG. 6A, the n-GaAs substrate 1 is prepared and then-GaAs layers and the n-AlAs layers are epitaxially and alternatelygrown by molecular beam epitaxy on the top surface of the n-GaAssubstrate 1 until the five periods of the alternating laminations of then-GaAs layers and the n-AlAs layers are formed to serve as the bottomdistributed Bragg reflector mirror 2. The n-Al₀.3 Ga₀.7 As epitaxiallayer serving as the bottom cladding layer 3 is grown by molecular beamepitaxy on the top surface of the bottom distributed Bragg reflectormirror 2. The non-doped In₀.2 Ga₀.8 As epitaxial layer serving as theactive layer 4 is grown on the top surface of the bottom cladding layer3 by molecular beam epitaxy. The p-Al₀.3 Ga₀.7 As epitaxial layerserving as the top cladding layer 5 is grown on the top surface of theactive layer 4 by molecular beam epitaxy. The p-GaAs layers and thep-AlAs layers are epitaxially and alternately grown by molecular beamepitaxy on the top surface of the top p-doped cladding layer 5 until thetwelve periods of the alternating laminations of the p-GaAs layers andthe p-AlAs layers are 6 formed to serve as the p-doped top distributedBragg reflector mirror thereby a vertical-to-surface transmissionelectro-photonic device substrate 7 is completed.

With reference to FIG. 6B, a photo-resist film is formed on a topsurface of the p-doped top distributed Bragg reflector 6 in thevertical-to-surface emitting laser substrate 7. The photo-resist film issubjected to patterning to form a photo-resist pattern within apredetermined area to be used for ion-beam etching. The photo-resistpattern has a horizontal size of 10 micrometers square.

With reference to FIG. 6C, except for the bottom one or two periods ofthe p-GaAs layers and the p-AlAs layers 6, the alternating laminationsof the p-GaAs/AlAs multiple layers 6 to act as the p-doped topdistributed Bragg reflector mirror is selectively removed by a reactiveion-etching 9 using a chlorine gas and the photo-resist pattern 8 in avertical downward direction until the bottom one or two pairs of thep-GaAs/AlAs layers 6 is exposed so that the mesa structure 10 of thep-doped top distributed Bragg reflector 6 is defined under thephoto-resist pattern 8. The mesa structure has a rectangular definition.After the selective removal of the alternating laminations of thep-GaAs/AlAs layers 6, the active layer 4 exists at a depth ofapproximately 5000 angstroms from the exposed surface of the bottom oneor two p-GaAs/AlAs layers.

With reference to FIG. 6D, after the reactive ion-etching to form themesa structure 10 of the top distributed Bragg reflector mirror 6, thephoto-resist pattern 8 is removed. A photo-resist material 16 having ahigh viscosity coefficient is applied on an entire surface of the deviceuntil the mesa structure 10 of the top distributed Bragg reflectormirror is at least embedded in the photo-resist material 16 with thehigh viscosity coefficient.

With reference to FIG. 6E, the applied photo-resist material 16 issubjected to a reactive ion-etching 17 with use of oxygen to be removeduntil the top surface of the mesa structure 10 of the top distributedBragg reflector mirror is exposed.

With reference to FIG. 6F, a negative type photo-resist 18 is applied onan entire surface of the device to overlay the exposed surface of thetop of the mesa structure 10 of the top distributed Bragg reflectormirror and the surface of the remaining photo-resist material 16 havingthe high viscosity efficiency. The negative type photo-resist 18 isrelatively thick and has a resistivity to any ion-implantation.

With reference to FIG. 6G, the negative type photo-resist 18 issubjected to a patterning to form a negative photo-resist pattern 19acting as a mask for subsequent ion-implantation to form a highresistive region. The negative photo-resist pattern 19 has apredetermined horizontal size. For example, the negative photo-resistpattern 19 has a square definition whose horizontal length is 30micrometers in the plane view. The negative photo-resist pattern 19 isexpanded in the horizontal direction by 10 micrometers from the mesastructure 10 at each side. After a post baking, only the remainingphoto-resist material 16 having the high viscosity efficiency iscompletely removed by use of a developing solution for a positive typephoto-resist. The negative type photo-resist pattern 19 having a largerhorizontal size than a horizontal size of the mesa structure 10 isformed on the top of the mesa structure 10 of the top distributed Braggreflector mirror.

With reference to FIG. 6H, an ion-implantation 11 of the proton in anoblique direction is accomplished by use of the negative photo-resistpattern 19 during a rotation of the substrate 1 to form a high resistiveregion of a proton implanted region. The oblique direction for theion-implantation 11 has an included angle of 14° to a level surface ofthe active layer 4, namely an included angle of 76° to a vertical axis.The ion-implantation 11 of proton is accomplished under the conditionsthat an energy of implantation is 300 KeV and a dose of is 5×10¹⁴ cm⁻².The high resistive region 12 comprising the proton implanted region hasa desired proton concentration profile along a vertical direction, whichhas a peak at the depth of approximately 5000 angstroms from the exposedsurface of the bottom two pairs of the p-GaAs/AlAs layers 6. Namely, theproton concentration profile of the high resistive region 12 has a peakat the depth at which the active layer 4 exists. The oblique directionion-implantation forms the high resistive region 12 of the protonimplanted region which defines the light emitting area or the carrierinjection area of the active layer 4 in which the carrier injection areacorresponds to an area in which the mesa structure 10 is formed in theplane view. The oblique direction ion-implantation also forms the highresistive region 12 of the proton implanted region which defines theunited inverse circular truncated cone definition of the p-type topcladding region and the bottom two pairs of the p-GaAs/AlAs layers. Theoblique direction ion-implantation also forms the high resistive region12 of the proton implanted region which defines the circular truncatedcone definition of the n-type bottom cladding region.

Cotrary to the vertical downward ion-implantation for formation of thehigh resistive proton implanted region in the above prior arts, in thisoblique angle ion-implantation, proton is implanted into the highresistive region 12 neither through any part of the p-type top claddingregion having the inverse circular truncated cone definition nor throughany part of the p-GaAs/AlAs layers having the united inverse circulartruncated cone definition, which could readily be understood from FIG.6H. Needless to say, after the proton implantation, none of protonexists not only in the inverse circular truncated cone definition of thep-type top cladding region and the p-GaAs/AlAs layers but also in thelight emitting area or the carrier injection area as well as in thecircular truncated cone definition of the n-type bottom cladding region.The inverse circular truncated cone and circular truncated conedefinitions of the p-type top cladding region and the p-GaAs/AlAs layersand the n-type bottom cladding region are completely free from anyexposure of the ion-implantation of proton and thus the above regionsare able to have extremely low electrical resistivities. The carriers orelectrons and holes are able to flow through the inverse and non-inversecircular truncated cone definition regions of the p-type top and n-typebottom cladding regions without any experience of a potential barrier.

The description of the fabrication processes of the novel device will bedirected to a relationship between the horizontal size of the negativetype photo-resist pattern 19 and the oblique angle for the obliquedirectional ion-implantation to form the high resistive region 12. FromFIG. 7, it could be understood that the oblique angle and the horizontalsize of the negative photo-resist pattern 19 for the oblique directionalion-implantation are respectively defined to comply with the followingconditions.

    b=(2×(W.sub.1 +W.sub.2)/W.sub.2)+a

    θ=tan.sup.-1 (W.sub.2 +x)

where "b" is the horizontal size of the negative type photo-resistpattern 19, θ is the oblique angle as an included angle to the levelline, "W₁ " is a height of the mesa structure 10. "W₂ " is the depth ofthe active layer 4 from the flat surface of the device around the mesastructure 10, "a" is the horizontal size of the light emitting area orthe carrier injection area of the active layer 4, which corresponds to ahorizontal size of the mesa structure 10 and "x" is the horizontaldistance of an edge of the top surface of the inverse circular truncatedcone definition region from the side of the mesa structure 10.

Whereas in the above descriptions the above n-type bottom and p-type topcladding regions 3 and 5 have the circular truncated cone and theinverse circular truncated cone definitions, modifications of thedefinitions thereof are acceptable. For example, the externaldefinitions of the n-type bottom and the p-type top cladding regions maybe a truncated pyramid and an inverse truncated pyramid respectively,for example, a quadrilateral truncated pyramid and an inversequadrilateral truncated pyramid. In this case, the oblique directionalion-implantation of proton is accomplished by use of the negativephoto-resist pattern 19 during a stationary state of the device. Theoblique directional ion-implantation of proton is discontinued during arotation of the substrate 1 by a predetermined angle. The above twosteps are repeated in required times. The oblique direction for theion-implantation 11 has an included angle of 14° to a level surface ofthe active layer 4, namely an included angle of 76° to a vertical axis.The ion-implantation 11 of proton is accomplished under the conditionsthat an energy of implantation is 300 KeV and a dose of is 5×10¹⁴ cm⁻².The high resistive region 12 comprising the proton implanted region hasa desired proton concentration profile along a vertical direction, whichhas a peak at the depth of approximately 5000 angstroms from the exposedsurface of the bottom two pairs of the p-GaAs/AlAs layers 6. Namely, theproton concentration profile of the high resistive region 12 has a peakat the depth at which the active layer 4 exists. The oblique directionion-implantation forms the high resistive region 12 of the protonimplanted region which defines the light emitting area or the carrierinjection area of the active layer 4 in which the carrier injection areacorresponds to an area in which the mesa structure 10 is formed in theplane view. The oblique direction ion-implantation also forms the highresistive region 12 of the proton implanted region which defines theunited inverse truncated pyramid definition of the p-type top claddingregion and the bottom two pairs of the p-GaAs/AlAs layers. The obliquedirection ion-implantation also forms the high resistive region 12 ofthe proton implanted region which defines the truncated pyramiddefinition of the n-type bottom cladding region.

The description will be back to the fabrication process for the devicefrom the modifications thereof.

With reference to FIG. 6I, the negative photo-resist pattern 19 isremoved. An n-electrode 13 is selectively provided at a peripheralposition on a bottom surface of the n-GaAs substrate 1 so that then-electrode 13 does not cover any light or laser beam emitting area.Alternatively, n-electrode 13 may be designed to have a window throughwhich the light or laser beam is able to be transmitted.

With reference to FIG. 6J, a p-electrode 14 is formed to cover not onlythe mesa structure 10 of the top distributed Bragg reflector mirror 6and its adjacent portions. The p-electrode 14 is provided to cover atleast the exposed top surface of the two pairs of the p-GaAs/AlAs layers6 in which the exposed surface is not covered by the mesa structure ofthe top distributed Bragg reflector mirror 6. The p-electrode 14 isprovided further to cover an edge portion of the top surface of the highresistive region 12.

The negative carriers or electrons may be able to be injected from then-electrode 13 into the active layer 4 in the carrier injection areathrough the circular truncated cone definition region of the n-typebottom cladding region in which the electrons have experienced nopotential barrier. Further, the positive carrier or holes may also beable to be injected from the p-electrode 14 at its peripheral portion ofthe mesa structure 10 into the active layer 4 in the carrier injectionarea through the inverse circular truncated cone definition region ofthe p-type bottom cladding region in which the holes have experienced nopotential barrier. It is much more important for reduction of the deviceelectrical resistance that the holes have experienced no potentialbarrier during flowing through the inverse circular truncated conedefinition region of the p-type top cladding region since holes have amuch larger effective mass than an effective mass of electrons.

The novel vertical-to-surface transmission electro-photonic device,therefore, has the following advantages. A large part of the injectioncarriers or holes flows from the p-electrode at the peripheral portionof the mesa structure 10 through the ion-free inverse circular truncatedcone definition region toward the light emitting area in the activelayer 4 and receives the carrier confinement by the high resistiveregions 12 of the proton implanted regions. Namely, a large part of theinjection carriers flows not through the mesa structure 10.

From the above, it could readily be appreciated that the above ion-freeinverse circular truncated cone definition region defined by the obliqueangle ion-implantation is able to permit a remarkable reduction of theelectrical resistance of the vertical-to-surface transmissionelectro-photonic device. The above desired definition of the highresistive region 12 is also able to permit the lateral carrierconfinement which provides the high density carrier injection into theactive layer 4. The high density carrier injection provides a stronglaser beam emission from the active layer 4 in the vertical direction tothe surface thereof. This leads to a remarkable improvement in theelectronic-optical conversion efficiency of the vertical-to-surfacetransmission electro-photonic device. The novel vertical-to-surfacetransmission electro-photonic device further is able to show remarkablyimproved excellent properties in an external differential quantumefficiency and a threshold current due to that the device having anextremely low electrical resistance is free from any problems caused bya heat.

A second embodiment according to the present invention provides a novelvertical-to-surface transmission electro-photonic device with a mesastructure, a novel structure of which will hereinafter be described indetail with reference to FIGS. 8 and 9.

FIG. 8 illustrates a fabrication step involved in a fabrication methodfor a novel vertical-to-surface transmission electro-photonic device ofthe second embodiment according to the present invention.

As illustrated in FIG. 9, a novel vertical-to-surface transmissionelectro-photonic device has a semiconductor substrate 1 which is made ofan n-GaAs semiconductor compound. A bottom distributed Bragg reflectormirror 2 is provided on a top surface of said n-GaAs semiconductorsubstrate 1. The bottom distributed Bragg reflector mirror 2 comprisesn-GaAs layers and n-AlAs layers which are alternately laminated. Thealternating lamination may, for example, comprise four periods of then-GaAs layers and n-AlAs layers, each of which has a thicknesscorresponding to a quater of a medium wavelength. The n-GaAs layers andn-AlAs layers have a relatively large difference to those refractiveindexes due to those compositional discontinuity to serve as thedistributed Bragg reflective mirror for reflecting a light or laserpropagated in a vertical direction. The n-GaAs layers and n-AlAs layerswith the large difference in the refractive index have also a relativelylarge discontinuity in the energy band gap due to the compositionaldiscontinuity.

A bottom cladding region 3 made of n-Al₀.3 Ga₀.7 As is provided on a topsurface of the bottom distributed Bragg reflector mirror 2. Asillustrated in FIG. 5, the bottom n-Al₀.3 Ga₀.7 As cladding region has adefinition of a circular truncated cone whose horizontal size ordiameter is gradually and proportionally minimized toward an upperdirection. The circular truncated cone definition of the bottom claddingregion is completely free from any exposure of an ion-implantation for aformation of a high resistive region for a lateral carrier confinement.Thus, the bottom cladding region having the circular truncated conedefinition has an extremely low electrical resistance through which anycarrier or electrons are able to flow without any experience of apotential barrier.

An active layer 4 is formed on a top surface of the bottom claddingregion 3. The active layer 4 comprises an i-In₀.2 Ga₀.8 As layer whichforms a quantum well structure. The active layer 4 is defined in ahorizontal carrier injection area which corresponds to the top surfacearea of the bottom cladding region 3.

A top cladding region 5 made of p-Al₀.3 Ga₀.7 As is provided on a topsurface of the active layer defined in the carrier injection area. Asillustrated in FIG. 5, the top p-Al₀.3 Ga₀.7 As cladding region 5 has adefinition of an inverse circular truncated cone whose horizontal sizeor diameter is gradually and proportionally enlarged toward the upperdirection. The inverse circular truncated cone definition of the topcladding region 5 is completely free from any exposure of anion-implantation for a formation of a high resistive region for alateral carrier confinement. Notwithstanding in the prior art the topcladding layer around the mesa structure is unavoidably exposed to theion-implantation thereby the top region unavoidably has a somewhat highelectrical resistivity. In contrast, in the second embodiment accordingto the present invention, the ion-free top cladding region having theinverse truncated cone definition is able to have an extremely lowelectrical resistivity. Since the bottom cladding region having thecircular truncated cone definition has an extremely low electricalresistance, any carrier or holes are able to flow without any experienceof a potential barrier. Since holes as the carrier in the p-type topcladding region 5 have a much larger effective mass than an effectivemass of electrons as carriers in the n-type bottom cladding region 3,the extremely low resistivity of the top cladding region having theinverse circular truncated cone definition is much more important forsecuring a desired low electrical resistance of the device. This is veryimportant to simultaneously secure both a high efficiency in theelectronic-optical conversion and a very low device resistance.

An oblique angle of a side surface of the inverse circular truncatedcone definition may be a desired angle to match various conditions. Theoblique angle of the side surface of the inverse circular truncated conedefinition of the top cladding region is defined by an oblique angle ofan oblique direction ion-implantation for a formation of a highresistive region 12 for a lateral carrier confinement. The highresistive region may comprises a proton implanted region. The highresistive region 12 acting for the lateral carrier confinement isprovided to define the inverse circular truncated cone definition of thep-type top cladding region 5, the carrier injection area of the activelayer 4 and the circular truncated cone definition of the n-type bottomcladding region 3.

Four pairs of p-GaAs/AlAs layers may be provided on the top surface ofthe p-type top cladding region 5 having the inverse circular truncatedcone definition. The four pairs of the p-GaAs/AlAs layers have anexternal definition like a flat inverse circular truncated cone smoothlyunited with the inverse circular truncated cone definition of the p-typetop cladding region 5. The p-type top cladding region 5 and the fourpairs of the p-GaAs/AlAs layers constitute a united inverse circulartruncated cone as illustrated in FIG. 9. Then, a side surface of theflat inverse circular truncated cone definition is defined by the highresistive region 12.

Alternatively, a single pair of p-GaAs/AlAs layer may be provided on thetop surface of the p-type top cladding region 5 having the inversecircular truncated cone definition whereas an illustration is omitted.The single pair of the p-GaAs/AlAs layers has an external definitionlike the inverse circular truncated cone smoothly united with theinverse circular truncated cone definition of the p-type top claddingregion 5. The p-type top cladding region 5 and the single pair of thep-GaAs/AlAs layers constitute a united inverse circular truncated coneas illustrated in FIG. 9.

Alternatively, no pair p-GaAs/AlAs layers may be provided on the topsurface of the p-type top cladding region 5 having the inverse circulartruncated cone definition whereas an illustration is omitted.

Whereas in the above descriptions the above n-type bottom and p-type topcladding regions 3 and 5 have the circular truncated cone and theinverse circular truncated cone definitions, modifications of thedefinitions thereof are acceptable. For example, the externaldefinitions of the n-type bottom and the p-type top cladding regions maybe a truncated pyramid and an inverse truncated pyramid respectively,for example, a quadrilateral truncated pyramid and an inversequadrilateral truncated pyramid.

A horizontal size or diameter of the inverse truncated pyramid of thep-type top cladding region 5 is gradually and proportionally enlargedtoward the upper direction. The inverse truncated pyramid definition ofthe top cladding region 5 is completely free from any exposure of anion-implantation for a formation of a high resistive region for alateral carrier confinement. Notwithstanding in the prior art the topcladding layer around the mesa structure is unavoidably exposed to theion-implantation thereby the top cladding layer over the taperingportion of the high resistive region unavoidably has a somewhat highelectrical resistivity. In contrast, the modification of the secondembodiment according to the present invention, the ion-free top claddingregion having the inverse truncated cone definition is able to have anextremely low electrical resistivity. Since the bottom cladding regionhaving the truncated pyramid definition has an extremely low electricalresistance, any carrier or holes are able to flow without any experienceof a potential barrier. Since holes as the carrier in the p-type topcladding region 5 have a much larger effective mass than an effectivemass of electrons as carriers in the n-type bottom cladding region 3,the extremely low resistivity of the top cladding region having theinverse truncated pyramid definition is much more important for securinga desired low electrical resistance of the device. This is veryimportant to simultaneously secure both a high efficiency in theelectronic-optical conversion and a very low device resistance.

An oblique angle of a side surface of the inverse truncated pyramiddefinition may be defined to match various conditions. The oblique angleof the side surface of the inverse truncated pyramid definition of thetop cladding region is defined by an oblique angle of an obliquedirection ion-implantation for a formation of a high resistive region 12for the lateral carrier confinement. The high resistive region maycomprises a proton implanted region. The high resistive region 12 actingfor the lateral carrier confinement is provided to define the inversetruncated pyramid definition of the p-type top cladding region 5, thecarrier injection area of the active layer 4 and the truncated pyramiddefinition of the n-type bottom cladding region 3.

Four pairs of p-GaAs/AlAs layers may be provided on the top surface ofthe p-type top cladding region 5 having the inverse truncated pyramiddefinition. The four pairs of the p-GaAs/AlAs layers have an externaldefinition like a flat inverse truncated pyramid smoothly united withthe inverse truncated pyramid definition of the p-type top claddingregion 5. The p-type top cladding region 5 and the four pairs of thep-GaAs/AlAs layers constitute a united inverse truncated pyramid asillustrated in FIG. 9. Then, a side surface of the flat inversetruncated pyramid definition is defined by the high resistive region 12.

The description will hereinafter be back to the second embodiment fromthe modifications thereof.

A top distributed Bragg reflector mirror 6 comprises p-GaAs layers andp-AlAs layers which are alternately laminated. The alternatinglamination may, for example, comprise thirteen periods of the p-GaAslayers and p-AlAs layers, each of which has the thickness correspondingto a quater of the medium wavelength. The top distributed Braggreflector mirror 6 is formed on a predetermined area in a top surface ofthe four pairs p-GaAs/AlAs layers having the inverse circular truncatedcone definition in which the predetermined area corresponds to the lightemitting area or the carrier injection area of the active layer 4 thetop cladding layer 5 in a plane view. The top distributed Braggreflector mirror 6 has a mesa structure 10 which comprises a bottomtrapezoid-shaped section including five pairs of the alternatinglaminations of the p-GaAs/AlAs layers and a top rectangular-shapedsection including seven pairs of the alternating laminations of thep-GaAs/AlAs layers. The p-GaAs layers and p-AlAs layers have arelatively large difference in those refractive indexes due to a largecompositional discontinuity to serve as the distributed Bragg reflectivemirror. The p-GaAs layers and p-AlAs layers with the large difference inthe refractive index have also a relatively large energy band gapdiscontinuity due to the large compositional discontinuity.

As modifications of the second embodiment, the above number of theperiods for the n-type bottom and the p-type top distributed Braggreflector mirrors may be changeable to much various conditions.

A p-electrode 14 is formed to cover not only the mesa structure 10 ofthe top distributed Bragg reflector mirror 6 and its adjacent portions.The p-electrode 14 is provided to cover at least the exposed top surfaceof the four pairs of the p-GaAs/AlAs layers 6 in which the exposedsurface is not covered by the mesa structure of the top distributedBragg reflector mirror 6. The p-electrode 14 is provided further tocover an edge portion of the top surface of the high resistive region12. An n-electrode 13 is selectively provided at a peripheral positionon a bottom surface of the n-GaAs substrate 1 so that the n-electrode 13does not cover any light or laser beam emitting area. Alternatively,n-electrode 13 may be designed to have a window through which the lightor laser beam is able to be transmitted.

The description of the second embodiment and some modifications thereofwill subsequently be directed to fabrication processes for the abovenovel vertical-to-surface transmission electro-photonic device.

With reference to FIG. 10A, the n-GaAs substrate 1 is prepared and then-GaAs layers and the n-AlAs layers are epitaxially and alternatelygrown by molecular beam epitaxy on the top surface of the n-GaAssubstrate 1 until the four periods of the alternating laminations of then-GaAs layers and the n-AlAs layers are formed to serve as the bottomdistributed Bragg reflector mirror 2. The n-Al₀.3 Ga₀.7 As epitaxiallayer serving as the bottom cladding layer 3 is grown by molecular beamepitaxy on the top surface of the bottom distributed Bragg reflectormirror 2. The non-doped In₀.2 Ga₀.8 As epitaxial layer serving as theactive layer 4 is grown on the top surface of the bottom cladding layer3 by molecular beam epitaxy. The p-Al₀.3 Ga₀.7 As epitaxial layerserving as the top cladding layer 5 is grown on the top surface of theactive layer 4 by molecular beam epitaxy. The p-GaAs layers and thep-AlAs layers are epitaxially and alternately grown by molecular beamepitaxy on the top surface of the top p-doped cladding layer 5 until thethirteen periods of the alternating laminations of the p-GaAs layers andthe p-AlAs layers are 6 formed to serve as the p-doped top distributedBragg reflector mirror thereby a vertical-to-surface transmissionelectro-photonic device substrate 7 is completed.

With reference to FIGS. 10B and 10C, a photo-resist film is formed on atop surface of the p-doped top distributed Bragg reflector 6 in thevertical-to-surface emitting laser substrate 7. The photo-resist film issubjected to patterning to form a photo-resist pattern within apredetermined area to be used for ion-beam etching. The photo-resistpattern 8 has a horizontal size of 10 micrometers square. Except for thebottom one or two periods of the p-GaAs layers and the p-AlAs layers 6,the alternating laminations of the p-GaAs/AlAs multiple layers 6 to actas the p-doped top distributed Bragg reflector mirror is selectivelyremoved by a reactive ion-etching 9 using a chlorine gas and thephoto-resist pattern 8 in a vertical downward direction until the bottomsixth pair of the p-GaAs/AlAs layers 6 is exposed so that the mesastructure 10 of the p-doped top distributed Bragg reflector 6 is definedunder the photo-resist pattern 8. As illustrated in FIG. 10C, the mesastructure comprises a single rectangular-shaped section.

With reference to FIG. 10D, after the reactive ion-etching to form themesa structure 10 of the top distributed Bragg reflector mirror 6, thephoto-resist pattern 8 is not removed. The remaining p-GaAs/AlAsmultiple layers 6 is further subjected to a wet etching 18 with use of aphosphoric acid system etchant so that the remaining p-GaAs/AlAsmultiple layers 6 is isotopic etched until the bottom first pair of thep-GaAs/AlAs layers 6 remains.

As illustrated in FIG. 10E, the wet etching defines the p-GaAs/AlAsmultiple layers 6 into the definition which comprises the bottomtrapezoid-shaped portion including the five pairs of the p-GaAs/AlAslayers 6 and the top rectangular-shaped portion including the sevenpairs of the p-GaAs/AlAs layers 6. The photo-resist pattern 8 isexpanded in the horizontal direction from the mesa structure 10 at eachside. The photo-resist pattern 8 has at least a larger horizontal sizethan a horizontal size of the rectangular-shaped portion of the mesastructure 10.

With reference to FIG. 10F, an ion-implantation 11 of proton in anoblique direction is accomplished by use of the photo-resist pattern 8during a rotation of the substrate 1 to form a high resistive region ofa proton implanted region. The high resistive region 12 comprising theproton implanted region has a desired proton concentration profile alonga vertical direction having a peak at the depth at which the activelayer 4 exists. The oblique direction ion-implantation forms the highresistive region 12 of the proton implanted region which defines thelight emitting area or the carrier injection area of the active layer 4in which the carrier injection area corresponds to an area in which themesa structure 10 is formed in the plane view. The oblique directionion-implantation also forms the high resistive region 12 of the protonimplanted region which defines the united inverse circular truncatedcone definition of the p-type top cladding region and the bottom fourpairs of the p-GaAs/AlAs layers. The oblique direction ion-implantationalso forms the high resistive region 12 of the proton implanted regionwhich defines the circular truncated cone definition of the n-typebottom cladding region.

Cotrary to the vertical downward ion-implantation for formation of thehigh resistive proton implanted region in the above prior arts, in thisoblique angle ion-implantation, proton is implanted into the highresistive region 12 neither throght any part of the p-type top claddingregion having the inverse circular truncated cone definition nor throughany parts of the p-GaAs/AlAs layers having the united inverse circulartruncated cone definition, which could readily be understood from FIG.10F. Needless to say, after the proton implantation, none of protonexists not only in the inverse circular truncated cone definition of thep-type top cladding region and the p-GaAs/AlAs layers but also in thelight emitting area or the carrier injection area as well as in thecircular truncated cone definition of the n-type bottom cladding region.The inverse circular truncated cone and circular truncated conedefinitions of the p-type top cladding region and the p-GaAs/AlAs layersand the n-type bottom cladding region are completely free from anyexposure of the ion-implantation of proton and thus the above regionsare able to have extremely low electrical resistivities. The carriers orelectrons and holes are able to flow through the inverse and non-inversecircular truncated cone definition regions of the p-type top and n-typebottom cladding regions without any experience of a potential barrier.

Whereas in the above descriptions the above n-type bottom and p-type topcladding regions 3 and 5 have the circular truncated cone and theinverse circular truncated cone definitions, modifications of thedefinitions thereof are acceptable. For example, the externaldefinitions of the n-type bottom and the p-type top cladding regions maybe a truncated pyramid and an inverse truncated pyramid respectively,for example, a quadrilateral truncated pyramid and an inversequadrilateral truncated pyramid. In this case, the oblique directionalion-implantation of proton is accomplished by use of the photo-resistpattern 8 during a stationary state of the device. The obliquedirectional ion-implantation of proton is discontinued during a rotationof the substrate 1 by a predetermined angle. The above two steps arerepeated in required times. The high resistive region 12 comprising theproton implanted region has a desired proton concentration profile alonga vertical direction having a peak at the depth at which the activelayer 4 exists. The oblique direction ion-implantation forms the highresistive region 12 of the proton implanted region which defines thelight emitting area or the carrier injection area of the active layer 4in which the carrier injection area corresponds to an area in which themesa structure 10 is formed in the plane view. The oblique directionion-implantation also forms the high resistive region 12 of the protonimplanted region which defines the united inverse truncated pyramiddefinition of the p-type top cladding region and the bottom four pairsof the p-GaAs/AlAs layers. The oblique direction ion-implantation alsoforms the high resistive region 12 of the proton implanted region whichdefines the truncated pyramid definition of the n-type bottom claddingregion.

The description will be back to the fabrication process for the devicefrom the modifications thereof.

With reference to FIGS. 10G and 10H, the photo-resist pattern 8 isremoved. An n-electrode 13 is selectively provided at a peripheralposition on a bottom surface of the n-GaAs substrate 1 so that then-electrode 13 does not cover any light or laser beam emitting area.Alternatively, n-electrode 13 may be designed to have a window throughwhich the light or laser beam is able to be transmitted. A p-electrode14 is formed to cover the mesa structure 10 of the top distributed Braggreflector mirror 6. The p-electrode 14 is provided to cover at least theexposed top surface of the four pairs of the p-GaAs/AlAs layers 6 inwhich the exposed surface is not covered by the mesa structure of thetop distributed Bragg reflector mirror 6. The p-electrode 14 mayselectively be provided further to cover an edge portion of the topsurface of the high resistive region 12.

The negative carriers or electrons may be able to be injected from then-electrode 13 into the active layer 4 in the carrier injection areathrough the circular truncated cone definition region of the n-typebottom cladding region in which the electrons have experienced nopotential barrier. Further, the positive carrier or holes may also beable to be injected from the p-electrode 14 on the trapezoid-shapedportion of the mesa structure 10 into the active layer 4 in the carrierinjection area through the inverse circular truncated cone definitionregion of the p-type bottom cladding region and the four pairs of thep-GaAs/AlAs layers in which the holes have experienced a slightpotential barrier. It is much more important for reduction of the deviceelectrical resistance that the holes have experienced no potentialbarrier during flowing through the inverse circular truncated conedefinition region of the p-type top cladding region since holes have amuch large effective mass than an effective mass of electrons.

The novel vertical-to-surface transmission electro-photonic device ofthe second embodiment according to the present invention, therefore, hasthe following advantages. The fabrication steps of the second embodimentare more simple than the fabrication process of the first embodiment.

A large part of the injection carriers or holes flows from thep-electrode 14 on the trapezoid-shaped portion of the mesa structure 10through the ion-free inverse circular truncated cone definition regiontoward the light emitting area in the active layer 4 and receives thecarrier confinement by the high resistive regions 12 of the protonimplanted regions. Namely, a large part of the injection carriers flowsnot through the mesa structure 10.

From the above, it could readily be appreciated that the above ion-freeinverse circular truncated cone definition region defined by the obliqueangle ion-implantation is able to permit a remarkable reduction of theelectrical resistance of the vertical-to-surface transmissionelectro-photonic device. The above desired definition of the highresistive region 12 is also able to permit the lateral carrierconfinement which provides the high density carrier injection into theactive layer 4. The high density carrier injection provides a stronglaser beam emission from the active layer 4 in the vertical direction tothe surface thereof. This leads to a remarkable improvement in theelectronic-optical conversion efficiency of the vertical-to-surfacetransmission electro-photonic device. The novel vertical-to-surfacetransmission electro-photonic device further is able to show remarkablyimproved excellent properties in an external differential quantumefficiency and a threshold current due to that the device having anextremely low electrical resistance is free from any problems caused bya heat.

Whereas modifications of the present inventions will no doubt beapparent to a person having ordinary skill in the art, to which theinvention pertains, it is to be understood that the embodiments shownand described by way of illustrations are by no means intended to beconsidered in a limiting sense. Accordingly, it is to be intended tocover by claims all modifications of the present invention which fallwithin the spirit and scope of the invention.

What is claimed is:
 1. A vertical-to-surface transmissionelectro-photonic semiconductor device comprising:a semiconductorsubstrate; a bottom light reflective mirror formed on said semiconductorsubstrate for reflecting a light propagating in a vertical direction; anintermediate multilayer structure formed on said bottom light reflectivemirror, said intermediate multilayer structure including top and bottomepitaxial cladding layers sandwiching a quantum well active layer whichshows a photoluminescence when carriers are injected into said activeregion, said top and bottom epitaxial cladding layers and said activelayer being surrounded by both an impurity implanted region within whichan impurity is implanted and a semiconductor crystal-damaged regionoverlying said impurity implanted region and said semiconductorcrystal-damaged region having a high resistivity increased by crystaldamages due to the fact that the impurity has passed therethrough forthe purpose of an implantation of impurities into said impurityimplanted region; a top light reflective mirror of a mesa structureformed on said intermediate multilayer structure for reflecting a lightpropagating in a vertical direction; and an electrode comprising a firstpart provided on said mesa structure and a second part provided on aperipheral region of said mesa structure; wherein said semiconductorcrystal-damaged region and said impurity implanted region are formed byan impurity implantation at a predetermined oblique angle so that, justunder said second part of said electrode, a region free of any crystaldamage in said epitaxial cladding layer is positioned and further saidsemiconductor crystal-damaged region and said impurity implanted regionhave a linearly sloped boundary extending at said oblique angle from afirst position at edges of said second part of said electrode to asecond position having the same level to said active layer andpositioned just under vertical side walls of said semiconductor mesastructure to restrict a usable area of said active layer within the samearea as said mesa structure to thereby allow that the majority of saidcarriers flows from said second part of said electrode through saidcrystal damage free region into said usable part of said active layerwithout passing through any part of said semiconductor crystal-damagedregion.
 2. The device as claimed in claim 1, wherein said oblique angleis approximately 14 degrees.
 3. The device as claimed in claim 1,wherein said impurity implantation in the oblique direction is carriedout by use of a mask provided on said mesa structure and having ahorizontal size larger than a size of said mesa structure wherein saidimpurity is implanted in said oblique direction.
 4. The device asclaimed in claim 3, wherein said oblique direction and said horizontalsize of said mask are determined to satisfy with the followingconditions:

    B=(2D(W.sub.1 +W.sub.2)/W.sub.2)+A; and

    β+tan.sup.-1 (W.sub.2 +D)

wherein B is the horizontal size of said mask, β is the oblique angle ofsaid oblique direction to said interfere of said active region, W₁ isthe vertical distance between a bottom of said mask and a top of saidtop epitaxial cladding layer, W₂ is the vertical distance between thefirst and second positions, A is the horizontal size of said activelayer and D is the horizontal distance between said first and secondpositions.
 5. The device as claimed in claim 3, wherein said impurityimplantation is carried out together with a rotation of said substratearound a center axis of said mesa structure.
 6. The device as claimed inclaim 1, wherein said mesa structure is defined by side wallsright-vertical to a surface of said substrate.
 7. The device as claimedin claim 1, wherein said mesa structure comprises:a first section beingwide-based to have various horizontal areas linearly increased in adirection from top to bottom; and a second section formed on said firstsection and defined by side walls right-vertical to a surface of saidsubstrate.
 8. The device as claimed in claim 1, wherein:said activelayer includes a non-doped In₀.2 Ga₀.8 As quantum well layer; and saidtop and bottom epitaxial cladding layers comprise Al₀.3 Ga₀.7 As layers.9. The device as claimed in claim 1, wherein each of said top and bottomreflective mirrors comprises alternating laminations of impurity-dopedGaAs layers and impurity-doped AlAs layers.
 10. The device as claimed inclaim 9, wherein said impurity-doped GaAs layers and impurity-doped AlAslayers have a thickness equal to a quarter of a medium wavelength oflight.